Underestimated color centers: Defects as useful reducing agents in lanthanide‐activated luminescent materials

Several inorganic hosts such as SrB 4 O 7 or certain nitrides are known to intrinsically stabilize Eu 2+ even upon doping with a Eu 3+ -based precursor and no employment of any reducing conditions during synthesis. Although this somewhat alchemistic concept has already been known for a long time in the field of phosphor synthesis, the mechanistic details of the intrinsic reductive action of certain hosts are only scarcely understood. Herein, we aim to demonstrate first clear experimental evidence that trapped charge carriers such as color centers can also act as potential redox partners to stabilize certain oxidation states of activators taking Eu-activated CsMgCl 3 and CsMgBr 3 as a representative example. Upon intentional doping with EuCl 3 and without employment of any reducing conditions during the synthesis, yet dominant cyan or green luminescence related to the presence of Eu 2+ ions was observed instead, respectively. Photoluminescence spectroscopy at 10 K revealed that this intrinsically occurring reduction could be clearly correlated to the presence of impurity-localized color centers. Although defects are otherwise typically undesired in phosphors, this study demonstrates that their role may be underestimated and they could be used on purpose in the preparation of selected inorganic phosphors.